B. L. Lipphardt

Impact of Assimilating Ocean Velocity Observations Inferred from Lagrangian Drifter Data Using the NCOM-4DVAR*

AbstractEulerian velocity fields are derived from 300 drifters released in the Gulf of Mexico by The Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) during the summer 2012 Grand Lagrangian Deployment (GLAD) experiment. These data are directly assimilated into the Navy Coastal Ocean Model (NCOM) four-dimensional variational data assimilation (4DVAR) analysis system in a series of experiments to investigate their impact on the model circulation. The NCOM-4DVAR is a newly developed tool for data analysis, formulated for weak-constraint data assimilation based on the indirect representer method. The assimilation experiments take advantage of this velocity data along with other available data sources from in situ and satellite measurements of surface and subsurface temperature and salinity. Three different experiments are done: (i) A nonassimilative NCOM free run, (ii) an assimilative NCOM run that utilizes temperature and salinity observations, and (iii) an assimilativ...

Reconstructing basin-scale Eulerian velocity fields from simulated drifter data

Abstract A single-layer, reduced-gravity, double-gyre primitive equation model in a 2000 km × 2000 km square domain is used to test the accuracy and sensitivity of time-dependent Eulerian velocity fields reconstructed from numerically generated drifter trajectories and climatology. The goal is to determine how much Lagrangian data is needed to capture the Eulerian velocity field within a specified accuracy. The Eulerian fields are found by projecting, on an analytic set of divergence-free basis functions, drifter data launched in the active western half of the basin supplemented by climatology in the eastern domain. The time-dependent coefficients are evaluated by least squares minimization and the reconstructed fields are compared to the original model output. The authors find that the accuracy of the reconstructed fields depends critically on the spatial coverage of the drifter observations. With good spatial coverage, the technique allows accurate Eulerian reconstructions with under 200 drifters deploy...

Ocean processes underlying surface clustering

Ageostrophic ocean processes such as frontogenesis, submesoscale mixed-layer instabilities, shelf break fronts, and topographic interactions on the continental shelf produce surface-divergent flows that affect buoyant material over time. This study examines the ocean processes leading to clustering, i.e., the increase of material density over time, on the ocean surface. The time series of divergence along a material trajectory, the Lagrangian divergence (LD), is the flow property driving clustering. To understand the impacts of various ocean processes on LD, numerical ocean model simulations at different resolutions are analyzed. Although the relevant processes differ, patterns in clustering evolution from the deep ocean and the continental shelf bear similarities. Smaller-scale ocean features are associated with stronger surface divergence, and the surface material clustering is initially dominated by these features. Over time, the effect of these small-scale features becomes bounded, as material traverses small-scale regions of both positive and negative divergence. Lower-frequency flow phenomena, however, continue the clustering. As a result, clustering evolves from initial small-scale to larger-scale patterns.

Statistical properties of the surface velocity field in the northern Gulf of Mexico sampled by GLAD drifters

The Grand LAgrangian Deployment (GLAD) used multiscale sampling and GPS technology to observe time series of drifter positions with initial drifter separation of O(100 m) to O(10 km), and nominal 5 min sampling, during the summer and fall of 2012 in the northern Gulf of Mexico. Histograms of the velocity field and its statistical parameters are non-Gaussian; most are multimodal. The dominant periods for the surface velocity field are 1–2 days due to inertial oscillations, tides, and the sea breeze; 5–6 days due to wind forcing and submesoscale eddies; 9–10 days and two weeks or longer periods due to wind forcing and mesoscale variability, including the period of eddy rotation. The temporal e-folding scales of a fitted drifter velocity autocorrelation function are bimodal with time scales, 0.25–0.50 days and 0.9–1.4 days, and are the same order as the temporal e-folding scales of observed winds from nearby moored National Data Buoy Center stations. The Lagrangian integral time scales increase from coastal values of 8 h to offshore values of approximately 2 days with peak values of 3–4 days. The velocity variance is large, O(1)m2/s2, the surface velocity statistics are more anisotropic, and increased dispersion is observed at flow bifurcations. Horizontal diffusivity estimates are O(103)m2/s in coastal regions with weaker flow to O(105)m2/s in flow bifurcations, a strong jet, and during the passage of Hurricane Isaac. The Gulf of Mexico surface velocity statistics sampled by the GLAD drifters are a strong function of the feature sampled, topography, and wind forcing

Negative potential vorticity lenses

Abstract The dynamical properties of inviscid fluid lenses, which rotate faster than the local Coriolis, were studied through numerical experiments. Such lenses have negative potential vorticity and generally require nonlinear dynamical balances. A solution scheme was used that preserved exactly the relevant nonlinearity of the Euler equations. The experiments described here were designed to determine how the lens evolution depends upon initial conditions. When the lens is isolated from background flows it was found that large initial anticyclonic particle spin tends to produce quasi-periodic behavior in the hydrodynamic fields. On the other hand, large initial deformation produced erratic solutions with both subinertial and superinertial frequency components. An investigation of the response of an initially circular lens to a steady background deforming flow was made. Two equilibrium solutions were found for this setting. The boundary between the equilibrium solutions and oscillating solutions was found to be extremely sensitive to initial conditions and other model parameters. However, as initial spin magnitude was increased, a decrease in the magnitude of environmental deformation required to achieve either equilibrium configuration was observed.

Research Overview of the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE)

ABSTRACT CARTHE (http://carthe.org/) is a Gulf of Mexico Research Initiative (GoMRI) consortium established through a competitive peer-reviewed selection process. CARTHE comprises 26 principal inve...

Coherent flows with near zero potential vorticity

Abstract The dynamics of small, intense inviscid fluid vortices rotating near the local Coriolis are examined through numerical experiments. These vortices have near zero potential vorticity (which is conserved) and are inherently nonlinear. Solutions for a quasigeostrophic vortex with no external forcing are contrasted with solutions where this balance was violated in specific ways. It was found that departure from quasigeostrophy can produce a rich variety of superinertial oscillations in the hydrodynamic fields. The effects of time-dependent external forcing are also considered. An initially circular vortex with zero potential vorticity was forced at its natural frequency (the inertial frequency). The vortex developed an elliptical structure and was stable, even when the magnitude of the forcing deformation was comparable to that of the unforced vortex. Of course, as the forcing amplitude was increased, the solution became unstable. These results are discussed in light of recent observations of small, coherent structures. It is suggested that these structures may contribute to the superinertial variability previously observed (and not fully explained) in several internal wave observations.

25 Years of Nonlinearity in Oceanography from the Lagrangian Perspective

The Lagrangian view of geophysical fluid dynamics relies heavily on nonlinear methods familiar to most geoscientists. These include identification of hyperbolic and elliptic regions in flow fields along with finite time and scale Lyapunov exponents and particle dispersion statistics. Here we identify elliptic and hyperbolic regions to study the life cycle of a large anticyclonic eddy in the Gulf of Mexico. Hyperbolic regions develop simultaneously down to 200m when the Loop Current sheds the ring. The ring’s migration across the Gulf is monitored by tracking the movement of its elliptic point. Its abrupt disappearance is the result of interaction with a nearby hyperbolic region, which exists to 150m. Some broader implications of this approach are discussed in the last section.